tell a true atmospheric CO2 story?
Z Jaworowski, T V Segalstad, & N Ono, 1992 227-284 Science
of Total Environment
ABSTRACT | INTRODUCTION
| EARLY STUDIES | THE
PERIOD OF HIGH CO2 READINGS | THE PERIOD OF
LOW CO2 READINGS | SHALLOW CORES
| DEEP CORES | MACRO-
AND MICRO-CRACKS | CONTAMINATED DRILLING
FLUIDS | CONTAMINATED ICE CORES | DISCREPANCIES
IN DEEP CORE DATA | AGE OF AIR IN ICE
| CHANGES IN THE CHEMICAL COMPOSITION
OF TRAPPED AIR | LIQUID IN ICE
| STRUCTURAL CHANGES AND PHYSICAL-CHEMICAL
PROCESSES | CONCLUSIONS | ADDENDUM
Until 1985 most studies of CO2 in gas inclusions in pre-industrial
ice indicated that CO2 concentrations (up to 2450 ppm) were higher
than the current atmospheric level. After 1985, lower pre-industrial
CO2 values were reported, and used as evidence for a recent man-made
CO2 increase. The errors in these revised values, however, are
of a similar magnitude to the apparent increase in atmospheric
CO2 level. The assumptions used in estimating lower CO2 values
in past atmospheres have been: no liquid phase in polar ice; younger
age of air than of ice due to free gas exchange between deep firn
and the atmosphere; and no change in composition of air inclusions.
These assumptions are shown to be invalid. Liquid saline water
exists in ice at low temperatures, even below -70ºC; airtight
ice layers are ubiquitous in Antarctic firn; and more than 20
physico-chemical processes operating in situ and in ice cores
contribute to the alteration of the chemical composition of air
inclusions. The permeable ice sheet with its capillary liquid
network acts as a sieve which redistributes elements, isotopes,
and micro-particles. Thirty-six to 100% of air recovered from
old ice is contaminated by recent atmospheric air during field
and laboratory operations. The value of -290 ppm, widely accepted
from glacier studies for the pre-industrial atmospheric CO2 level,
apparently results from: invalid assumptions; processes in ice
sheets; artifacts in ice cores; and arbitrary rejection of high
readings. To date, glaciological studies are not able to provide
a reliable reconstruction of either the CO2 level in pre-industrial
and ancient atmospheres or paleoclimates. Instead these studies
have led to a widely accepted false dogma of man-made climatic
warming. This dogma may have enormous negative impact on our common
More than 50 years ago, Callendar (1938) revived the hypothesis
of greenhouse warming due to man's activity, proposed by Arrhenius
(1896) four decades earlier. Callendar's was a pioneering paper
posing questions which have now become the subject of intense
discussion. Among these questions are those related to changes
in global temperature, the natural carbon cycle, and man's contribution
to both. Callendar may truly be regarded as the father of the
current paradigm on man-induced global warming.
In support of this hypothesis, Callendar (1938, 1940, 1958) used
his estimate of an increase in atmospheric CO2 from a 19th century
level of 292 ppm to about 325 ppm by 1956. He claimed that both
this increase and a 0.33ºC rise in global surface temperature
between 1880 and 1935 were caused by fossil fuel burning. A set
of 19th century data compiled by Fonselius et al (1956) indicated
that CO2 concentrations ranged from about 250 to 550 ppm (Fig
1). Callendar accepted an average concentration of 292 ppm for
the 19th century data by application of a selection method questioned
by Slocum (1955), who demonstrated that without such selection
these data average 335 ppm.
Slocum (1955) pointed out that, from a set of twenty-six 19th
century averages, Callendar rejected 16 that were higher than
the global average of 292 ppm, and only two that were lower. On
the other hand, from the 20th century set Callendar rejected three
averages that were lower than his global average of 317 ppm, and
none that was higher. This shows a bias in the selection method.
Five decades later the Callendar CO2 estimates are still used
and their reliability discussed (see eg Stanhill 1982; Waterman
1983; Wigley 1983).
|Fig 1. Average atmospheric CO2 concentrations
measured in the 19th and 20th centuries. Encircled are the
values used by Callendar. Redrawn after Fonselius et al. 1956.
Bottom area enlarged, detail RHS.
It is interesting to note that a tendency to select low values
for the CO2 concentration in the 19th century atmosphere still
exists. This is because the carbon cycle model simulations, which
are based on an assumption that the increase in atmospheric CO2
is due only to man's activity, require starting concentrations
even lower than 290 ppm to agree with current CO2 observations
at Mauna Loa (Keeling et al 1976; Wigley 1983; Siegenthaler and
Oeschger 1987), an active and strongly CO2 emitting volcano (Jaworowski
et al 1990a).
Wigley (1983) claimed that "the most compelling support
for a (low) 270 ppm pre-industrial CO2 level comes from direct
measurements of CO2 in the ice cores", and cited Neftel et
al (1982) in support of this statement. But no such evidence was
presented by Neftel et al (1982). Their data indicate rather a
decreasing trend during the last 2000 years. They found that CO2
concentrations in air bubbles from 150-year-old ice ranged from
300 to 2350 ppm.
Ironically, those who found CO2 concentrations of between 270
and 390 ppm in 280-year-old ice also preferred values close to
the lower end of the range, because these were "within the
range of the estimated (by Callendar) pre-industrial atmospheric
content of 290 ppm" (Berner et al 1978; for similar statements
see also Raynaud and Barnola 1985, and Pearman et al 1986).
Because of uncertainties in 29th century air measurements, studies
of CO2 in glacier ice became a cornerstone of the current greenhouse
warming edifice, and a basis for studies of the global carbon
cycle (eg Broecker et al 1985; Bolin et al 1989). It is astonishing
that these studies have been so credulously accepted (eg IPCC
1990), and were never critically evaluated, except by Jaworowski
et al (1990a). Thorough validation of these studies is much required
in view of the enormous cost which may unduly be imposed on society
by incorrect interpretation of their results. In this paper we
present a more detailed discussion of the reliability of these
Glaciers are often thought of as the cleanest parts of the Earth's
surface, providing a unique chemical record of past atmospheric
environments. Falling snow flakes collect atmospheric aerosols
containing particulate and gasious components of the atmosphere.
Deposited on the surface of glaciers, they are covered each year
by consecutive layers of snow, and form an annual ice stratification.
The air is trapped within snow crystals, in liquids at their surfaces,
and in the voids of the porous firn structure. The pores close,
and after transition of firn into ice, form completely occluded
air bubbles. But before this stage, the air in the firn is isolated
from the atmosphere by dense layers of ice. Such ice layers, sandwiching
the firn into airtight pockets, are ubiquitous both in temperate
glaciers and in polar ice sheets.
These characteristics of glaciers enable us to study changes
in atmospheric composition during the past hundreds and thousands
of years, provided that the concentrations of chemical species
recovered from the snow or ice samples are directly proportional
to their original atmospheric concentrations. Atmospheric components
associated with insoluble particulates usually meet this condition,
but gases may not.
The validity of current reconstructions of pre-industrial and
ancient atmospheres, based on CO2 analyses in polar ice, depends
on three speculative assumptions:
(1) that the age of the gases in the air bubbles is much lower
than the age of the ice in which they are entrapped (eg Oeschger
et al 1985);
(2) that "the entrapment of air in ice is essentially a mechanical
process of collection of air samples, which occurs with no differentiation
of gas components" (Oeschger et al 1985); and
(3) that the original air composition in the gas inclusions is
The main argument in support of the last two assumptions is another
assumption that no liquid phase occurs in the polar ice at a mean
annual temperature of -24ºC or less (Berner et al 1977; Raynaud
and Barnola 1985; Friedli et al 1986). This is why, after initial
research in the 1950s and 1960s, CO2 glacier studies were conducted
exclusively on the Greenland and Antarctic ice sheets where such
low temperatures exist. Over time the sampling sites were transferred
to increasingly cooler regions.
As will be seen in the discussion below, all these assumptions
are invalid in view of the following: (1) the ubiquitous presence
of ice layers in Antarctic snow, making the age of the entrapped
air similar to the age of the ice; (2) the presence of liquid
water in polar ice even at the lowest Antarctic temperatures;
and (3) the occurrence of physical and chemical phenomena in glacier
ice. Due to these phenomena, entrapment of air in ice is not just
a mechanical process, but one that leads to substantial chemical
and isotopic changes (Segalstad and Jaworowski, in prep) in the
composition of gas inclusions.
Determinations of CO2 in snow and ice were initiated at a small
glacier in Norway (Coachman et al 1956, 1958a, b), and then the
studies were continued in Greenland and Antarctica (Table 1).
In the first Antarctic study of Matsuo and Miyake (1966) an elegant
method of 13C isotopic dilution was used for CO2 determinations.
The precision of these determinations, with an analytical error
of +/- 0.002%, was never matched in later studies, which reported
errors usually ranging between +/- 0.2 and 3%.
Two important observations were made in these early studies.
It was found that the CO2 content of the air trapped in pre-industrial
and ancient ice is rather high, and has a very wide concentration
range of about 100-7400 ppm (Table 1). Even more important was
the finding that several physical and chemical processes (such
as melting, the presence of liquid brines in the capillary-like
interstitial voids, the presence of carbonates, over-pressure
in the air bubbles, and solid deposition of super-cooled fog,
combined with large differences in the solubility of different
gases in cold water, and mobility of CO2 in ice) lead to differentiation
of the original atmospheric ratios of N2, O2, Ar, and CO2, and
to depletion or enrichment of CO2 in the ice (Coachman et al 1958;
Hemmingsen 1959; Scholander et al 1961; Matsuo and Miyake 1966;
Raynaud and Delmas 1977).
In these early studies it was recognized that the liquid water
in glaciers may be the most important factor in this differentiation,
because the composition of atmospheric air (78.08% nitrogen, 20.95%
oxygen, 0.93% argon, and approximately 350 ppm by volume, carbon
dioxide) is different from the composition of air dissolved in
cold water. It was known at that time that, in such air, at 0ºC
and ambient pressure, the concentration of oxygen is 67% higher
than in the atmosphere (Scholander et al 1961; Hodgman et al 1962).
This is because the solubility of oxygen in cold water is 2.1
times higher than that of nitrogen. Argon has 2.4 times higher
solubility than nitrogen, and CO2 73.5 times higher (Weast et
al 1989). This explains why the air extracted from melt layers
in polar firn has extremely high concentrations of about 12,000
ppm CO2 (Stauffer et al 1985). This is also why the air bubbles
contain much lower concentrations of CO2 than the ice which encompasses
Three different methods of gas extraction were used, and they
produced different results. This is illustrated in Fig 2. It can
be seen that in air from the same section of a pre-industrial
ice core, after 7 hrs "wet" extraction of melted ice,
the CO2 concentration was up to about 1000 ppm, and it was 1.5
- 4.5 times higher than after [just] 15 min "wet" extraction.
The "dry" extraction, consisting in crushing or shaving
the ice samples at about -20ºC, produced results similar
to the 15 min "wet" extraction. The short "wet"
and "dry" extractions recovered about a half or less
of the total CO2 present in the ice.
Later papers showing the recent increase in CO2 atmospheric level
ignored or played down these early findings. It was tacitly assumed
in these papers that no exchange exists between the ice matrix
rich in CO2 and the gas occluded in the air bubbles.
THE PERIOD OF HIGH CO2
After 1980 most of the studies of CO2 in glaciers were carried
out on Greenland and Artarctic ice by Swiss and French research
groups; one core was studied in an Australian laboratory. A striking
feature of the data published until about 1985 is the high concentrations
of CO2 in air extracted from both pre-industrial and ancient ice,
often much higher than in the contemporary atmosphere (Table 1).
Fig. 2. Concentration of CO2 in a
90-cm long section of a Camp Century (Greenland) ice core.
The lower curve represents 15 min. "wet" extraction
from melted ice and "dry" extraction; the upper
curve 7 hours "wet" extraction. Redrawn after
Stauffer et al (1981)
For example, in 11 samples of about 185-year-old ice from Dye
3 (Greenland) an average CO2 concentration of 660 ppm was measured
in the air bubbles (using the "dry" extraction method),
with a range of 290 - 2450 ppm (Stauffer et al 1985). In a deep
ice core from Camp Century (Greenland), covering the last 40,000
years, Neftel et al (1982) found CO2 concentrations in the air
bubbles ranging between 273 and 436 ppm (average 327 ppm). They
also found that in an ice core of similar age from Byrd Station
(Antarctica) these concentrations ranged between 257 and 417 ppm.
Both these deep cores were heavily fractured and contaminated
with drilling fluid. Neftel et al (1982) arbitrarily assumed that
"the lowest CO2 values best represent the CO2 concentrations
of the originally trapped air".
Using the same dry extraction method, in the same segment of
an ice core from a depth of 1616.21m in Dye 3 (Greenland), Neftel
et al (1983) found a CO2 concentration of 773 ppm in the air bubbles.
Two years later, Stauffer et al (1985) reported only about half
of this concentration (410 ppm).
It appears from Table 1 that the change from high to low CO2
values reported for polar ice occurred in the middle of 1985.
THE PERIOD OF LOW CO2 READINGS
Since 1985, low concentrations, near a value of 290 ppm or below,
started to dominate the records. They were interpreted as indicating
"the CO2 increase during the last 150 years" and "overlapping
or adjacent to results from direct measurements on Mauna Loa started
in 1958" (Stauffer and Oeschger 1985).
Except for a cursory description of a few measurements in Greenland
and Antarctica (Stauffer and Oeschger 1985), there were six cores
analyzed for CO2, all from Antarctica, and in all of them much
lower values of CO2 were reported for pre-industrial and ancient
ice than during the previous Antarctic studies (Table 1). In all
these cores the CO2 contents were determined only in gas extracted
by the "dry" method from primary air bubbles or secondary
air cavities, a few months to 19 years after drilling. No information
was given on the total CO2 contents of the ice itself.
A striking feature of these studies is that the sampling was
started deep below the surface, and no data were presented on
the recent concentration of CO2 in firm and ice deposited in the
twentieth century. The results from these younger strata are essential
for estimates of temporal changes of COw levels, because they
could demonstrate whether the air trapped in the firm contains
the original atmospheric concentrations of CO2.
The only study in which frozen firn samples were sealed in airtight
containers before analysis demonstrated that this is not the case
(Raynaud and Delmas 1977). In the top 0-1 m layers of firn from
Vostok Station (mean annual temperature about -57ºC) and
Pionierskaya (mean annual temperature -37.6ºC), which represent
contemporary precipitation, the authors found a CO2 concentration
in the interstitial air of 240 and 160 ppm respectively (Table
1). This is much lower than in the present atmosphere, and similar
to concentrations in air recovered from Antarctic ice of pre-industrial
and ancient age. These lwo CO2 concentrations are due to various
processes in the ice sheet (discussed below) or in the firn samples.
A serious flaw of the Antarctic studies discussed here is a scanty
description of the cores, and the disregard of the methodological
details which bear on the interpretation of the analytical results.
We do not learn whether the cores were drilled thermally or mechanically
(Neftel et al 1985, 1988; Raynaud and Barnola 1985; Barnola et
al 1987). Even the diameters of the cores are not indicated. This
diameter is changing with time; first it increases due to relaxation
of the load pressure, and then shrinks cue to sublimation of the
core during the long time that elapses between its collection
and analysis. Up to 30% shrinking of the volume was reported for
an ice core stored for 16 years at a temperature of -20ºC
(Ng and Patterson 1981). Information on the structure, texture
and history of the cores is not presented. Outstanding in this
respect is a paper by Neftel et al (1985) in which no information
at all is given on the collection of a South Pole core. Some such
flaws are discussed below.
Important information on the temperature changes during collection
of the cores, their transportation from Greenland, Antarctica,
or storage facilities in the United States, to laboratories in
Europe or Australia, and during their long storage, are not given
at all. That such temperature changes occur may be inferred from
Pearman et al (1986) who found that parts of the Law Dome core
exhibited "post-coring melting". The Law Dome samples
exposed to melting yielded significantly lower CO2 concentrations
in air extracted by the "dry" method from the air bubbles.
This indicates that a proportion of the CO2 from air bubbles was
dissolved in meltwater, and by subsequent refreezing (at -80ºC)
of this water, was eliminated from the bubbles.
According to Etheridge et al (1988) the Siple core drilled during
the Antarctic summer of 1983/84 and used by Neftel et al (1985)
was exposed to melting, but the latter authors did not mention
this in their paper. Other cores from Siple also "melted
partially during shipment" (Alley and Bentley 1988). Increasing
the temperature of polar ice cores during transportation and handling
to near the melting point is probably not a rare phenomenon and
was often reported (eg Ng and Patterson 1981; Boutron and Patterson
1983; Legrand et al 1988), but not by CO2 students except Etheridge
et al (1988).
Two types of Antarctic cores were analyzed in the "low readings
period": three shallow cores representing pre-industrial
ice, and two deep cores reaching the ice layers deposited about
160,000 (Vostok core) and 50,000 (Byrd core) years ago. (We do
not discuss three 2-cm long sections of the South Pole core.)
Most of these cores were drilled many years before analysis, and
all were exposed to ambient atmospheres and physical and chemical
changes during drilling and storage. The results of these five
studies have been used as proof of the recent increase in atmospheric
CO2 and for reconstruction of its long-term variations (eg IPCC
1990). They deserve closer examination.
Samples from three approximately 108-473 m long cores of pre-industrial
ice were analyzed by Neftel et al (1975), Friedli et al (1986),
Raynaud and Barnola (1985), Pearman et al (1986) and Etheridge
et al (1988). The samples were collected at the Siple Station
(average annual temperature -24ºC), D-57 (average temperature
-32ºC) and Law Dome-5 (average temperature -22ºC), from
various depths starting at 68, 89 and 72 m respectively, below
the surface of the ice sheet (Table 2).
From the precipitation rate, Neftel et al (1985) estimated the
age of the ice in the Siple core at 68 m to be 1890 AD, but they
assumed that the air was trapped in this ice during 1962-1983
(ie the air was about 90 years younger). In the D-57 core, Raynaud
and Barnola (1985) found, at a depth of about 50 m, sulphur deposition
from the Tambora (Indonesia) volcanic explosion in 1815. The corresponding
precipitation rate was calculated to be 18 cm of ice equivalent.
Therefore the age of the ice at 89 m depth was abpit 1680 AD,
but the authors assumed that the air in this ice was trapped in
1940 AD (ie it was 260 years younger than the ice). For the Law
Dome-5 core, Pearman et al (1986) and Etheridge et al (1988) gave
neither the depth at which ice samples were collected nor the
age of the ice. According to a private communication by Etheridge
(1990) the youngest ice was collected at -72 m depth, and it was
deposited about 1896 AD. But they assumed that the age of air
in this ice was 70 years younger (ie it was trapped in 1966 AD).
The validity of these assumptions is discussed below.
The CO2 concentrations recovered from these three cores at the
starting depths were 328, 288 and 325 ppm respectively, and they
were more or less systematically decreasing with increasing depth
and pressure in the ice. This is most clearly seen from the data
for the Siple core (Fig 3) and for the Law Dome-5 core (Etheridge
et al, 1988). Only one explanation for these observations was
offered: fossil fuel burning, although the ice samples were not
from the twentieth century.
The results were interpreted as indicating "a rapid increase
in atmospheric CO2... during the second half of the nineteenth
century or early twentieth century" (Pearman et al 1986),
"the increase in atmosphericCO2 due to burning of fossil
fuels and probably due to the anthropogenic influence on the biosphere
over the nineteenth and twentieth century" (Raynaud and Barnola
1985), and that "atmospheric CO2 concentration around 1750
was 280 ppmv and had increased since, essentially because of human
factors, by 22.5% to 345 ppmv in 1984" (Neftel et al 1985).
Another explanation for the concentration gradient decreasing
with depth is considered more plausible. As may be seen in Fig
3, the CO2 concentration in gas recovered from air bubbles in
the Siple core, between a depth of 68 and 187 m depth, where the
age of the ice was 173 and 322 years BP respectively. At a similar
depth interval of 126 to 250 m in the Vostok core (age of ice
4050 and 9320 years BP) a similar decrease in CO2 concentration
of about 8% occurred.
The decrease in the CO2 content of ice of completely different
age but exposed to similar changes in load pressure id due to
three factors: differential diffusion and solubility, clathrate
formation of air components in the ice sheet, and micro-fracturing
of ice cores with increasing depth (see below).
Different diffusion rates of CO2 into the ice lattice and liquid
veins versus other air components is related to the lower molecular
volume and lower gas viscosity of CO2 than for N2, O2 and Ar.
Differential diffusion occurs both in the ice sheet and in the
ice cores in which the post-drilling structure disturbances are
related to the depth from which the cores were recovered.
The pressure in air bubbles in cores recovered from below about
100 m depth is less than the load pressure in the ice sheet (see
section Structural Changes and Physico-chemical Processes). The
total gas contents in these cores were about >= 25% lower than
in those from shallower depths. This is due to gas release from
air bubbles via micro-cracks developed in ice cores recovered
from below 100 m depth. Below 100 m the frequency of cracks increases
with depth. CO2 may escape through these micro-cracks at a different
rate than other gases. Disturbances in the ice structure are associated
with redistribution of the liquid phase and of the gases, which
leads to differentiation of the gas composition of the air bubbles.
The solubility of CO2 in cold water increases with decreasing
temperature, and at a faster rate than the solubilities of N2
and O2 increase. At higher pressure more CO2 than other air components
dissolves in the intercrystalline liquid (Enns et al 1965). Siegenthaler
et al (1988) found that the oxygen in CO2 gas inclusions of ice
from three Antarctic stations had exchanged isotopes and was close
to isotopic equilibrium with the ice. They calculated that a 10-molecular-layers
thick shell of H2O around each gas inclusion permits rapid equilibration
between CO2 and H2O. This mechanism is also valid for the firn
and ice strata discussed here.
In the region of load pressure between 5 and 9 bars the CO2 concentration
decreases more rapidly than at greater depths ie between 9 and
15 bars. The first steep decrease is probably due to the formation
of CO2 clathrate (solid-state hydrate: CO2+5.75H2O, which at temperatures
below -15ºC is expected to form at a pressure of 5 bars or
lower (Takenouchi and Kennedy 1965). We assume that the location
of CO2 clathrate is related to the presence of liquid water and
vapour in the ice structure, because free water molecules are
needed for its formation. N2 and O2 need much higher pressures
than CO2 (above 70 and 100 bars respectively) to enter the clathrate
phase at low temperatures (Miller 1969). When the pressure increases
above 5 bars, CO2 starts to enter the clathrate form, long before
N2 and O2. Thus differential diffusion, solubility, clathrate
formation, and cracking of ice makes depletion of CO2 from the
air bubbles in shallow ice cores roughly proportional to increasing
As mentioned before, the core samples studied by Pearman et al
(1986) were exposed to post-coring melting, and then to a temperature
of -80ºC. This certainly changed the composition of the gas
inclusions, making determinations unreliable for estimates of
past atmospheric CO2 levels.
Raynaud and Barnola (1985) and Neftel et al (1985) observed totally
different concentrations of CO2 in air bubbles from ice at the
starting depths, ie lower and higher, respectively, than in the
present atmosphere. However, both groups reached the same conclusion:
that their results demonstrate an anthropogenic increase in atmospheric
CO2. This suggests that there is bias in the interpretation of
Jaworowski et al (1990a) compared the range of analytical uncertainties
with the temporal differences in CO2 concentrations in ice used
in support of claims discussed above that the level of this gas
has increased in the atmosphere due to man's activity. This "CO2
glacier signal" was in the case of Raynaud and Barnola (1985)
17 ppm, in the case of Neftel et al (1985) 49 ppm, and in the
case of Pearman et al (1986) 13 ppm (Fig 4). According to Oeschger
et al (1985) the "errors" (at assumed 68% probability)
of single measurements of CO2 in air trapped in ice cores from
Greenland and Antarctica range between 11 and 24 ppm. At an assumed
95% probability, the "errors" of measurements reach
about 47 ppm (Fig 4). Thus the claimed signals of man-made CO2
increase are of the same magnitude as the range of uncertainty
In air bubbles from neighbouring 1.5 cm thick slices of an Antarctic
ice core, representing 6 months precipitation, the CO2 concentrations
differed by 32 ppm and by 25 ppm in the same slice (Barnola et
al 1983) (see discussions below).
It is clear from this comparison that the claims of a recent
increase in atmospheric CO2 content, based on glacier studies,
are not justified by the available data.
Carbon dioxide concentrations were studied in two deep cores from
Antarctica collected at the Vostok station (126.4 - 2077.5 m below
the surface) (Barnola et al 1987) and at the Byrd station (~600
to ~1900 m below the surface) (Neftel et al 1988). The assigned
age of the ice in these cores spanned from 4050 to 159,690 years
BP. The same assumptions as for shallow cores with respect to
the stability of the gas composition and air age were accepted
in these studies. The conclusions in these papers were based on
an unverified assumption that the CO2 variations observed in the
cores reflect the real changes in composition of the ancient atmosphere.
We challenge this assumption because new evidence indicates that
the ice in the polar sheets is not a single, solid-state phase
with bubbles in which the air composition is preserved indefinitely,
and because the elemental and isotopic composition of air in the
gas inclusions is changed by many physico-chemical processes in
the ice sheets and in the ice cores.
For the Vostok core, Barnola et al (1987) provided no information
as to how it was decontaminated from the drilling fluid, how deep
this fluid and meltwater penetrated the core, what was the vertical
and radial distribution of the micro- and macro-cracks and how
the authors countered this problem, which might seriously influence
their CO2 determinations. Instead, a general statement on "the
good core quality" is offered. This statement is in disagreement
with the reports of De Angelis et al (1987) and Petit et al (1990)
on the poor quality of this core.
There is also a lack of important information on the time elapsed
between the collection of the Vostok core and the CO2 analysis.
No information is provided on the post-drilling history of the
core samples, how they were preserved, how their temperature changed
during transportation, storage and analysis, and how much their
volume decreased due to sublimation and evaporation. The diameters
of the sections of the Vostok core analyzed by Boutron et al (1988)
of 9.1 and 10.5 cm were different than the original diameters
reported for various core depths by Kudryashov et al (1984a) of
8.8 and 11.1 cm. Sublimation leads to an important loss of mass
of the cores stored for longer times at about -20ºC. This
loss is neither controlled nor recorded, but is easily seen in
the form of hoar covering the surface of the cores and deposited
inside their envelopes. Sublimation will seriously influence the
isotopic composition of the ice (Segalstad and Jaworowski, in
There is neither information on the disappearance of air bubbles
in the ice sheet, reported at Vostok to occur below 800 m depth
(Korotkevich et al 1978), nor on their re-appearance as secondary
cavities in the bubble-free part of the core after its collection
and decompression (see discussion below). This, among other effects,
led to mixing of the gas from primary air bubbles with much larger
amounts of CO2 enclosed in the ice itself. There is no evidence
for the expectation that the gas entering the secondary cavities
may have the same composition as the gas in the primary bubbles,
or the original ancient atmospheric composition. Most of this
criticism is valid for descriptions of other cores studied in
Antarctica since 1985.
For the Byrd core, Neftel et al (1988) presented neither information
on, nor a reference to, the analytical methods used, preparation
of samples, and the effects of storage of the unsealed core sections
for 19 years "at different places" (Friedli et al 1984).
The data of Neftel et al (1988) show a maximum value for the CO2
concentration in air recovered from this core of about 290 ppm.
In a previous paper, much higher concentrations were reported
for the same core (Neftel et al 1982). The high readings recorded
by Neftel et al (1982) in the upper part of the Byrd core of 417
and 325 ppm are omitted in the Neftel et al (1988) paper. Figure
5 demonstrates this selection of the results.
A criterion for such data selection was the high CO2 concentration,
and its discussion by Neftel et al (1982, 1988) is rather fuzzy.
They stated that "in the larger samples (300g) contamination
[with drilling fluid] is almost inevitable and the measured CO2
concentrations tend to be higher than the air [sic!] originally
included in the ice". However, they did not measure CO2 in
the air bubbles and secondary gas cavities from such larger samples,
but only from 1g samples. These small samples were used to "avoid
samples with visible internal cracks". They stated that,
in the case of small (~1g) samples, their "small size meant
that [they were] probably uncontaminated and we conclude that
the lowest CO2 values best represent the CO2 concentrations of
the originally trapped air". But they recorded both the high
and the low CO2 concentrations in 1g samples, so their conclusion
is neither based on analytical evidence nor logical arguments.
As discussed below, 1g ice core samples may contain about 5 healed
macro-cracks. Visual inspection is not sufficient to distunguish
contaminated samples from uncontaminated ones, because invisible
micro-cracks containing drilling fluid are present in the inner
parts of the cores where no visible cracks were observed (Gow
and Williamson 1975; see also the section Macro- and Micro-cracks).
It is not possible to distinguish between contaminated and uncontaminated
samples on the basis of their size.
Neftel et al (1988) claimed that their data revealed 30% lower
CO2 values during glacial periods than during interglacial periods.
However, the lower values are from parts of the core where many
melt layers and important ice structure changes occurred (Gow
1970). The decrease in CO2 concentrations started at 1200 m depth,
where all original air bubbles disappear in the ice sheet. Here
the number of secondary cavities, an artifact appearing in the
relaxed ice cores, reached its maximum. It is from these secondary
cavities and not from the original air bubbles that the gas samples
with the lowest CO2 content were collected by Neftel et al (1988).
Neftel et al (1988) stated that, in the core, "no melt features
were observed", quoting Gow (1968a) in support. ButGow (1970)
found many "layers of variable thickness of refrozen meltwater
up to 10 mm thick" in this core. Apart from this, a distinctive
ice crust layering (up to about 10 layers per meter) was observed
in the firn strata, and below down to a depth of 240 m. The formation
of ice crusts involved local radiational melting (Gow 1970).
The Byrd core was contaminated with a drilling liquid composed
of a mixture of trichloroethylene and diesel oil (Gow 1971) and
with meltwater from thermal drilling. Neftel et al (1982, 1988)
did not mention any attempt to decontaminate the core. This drilling
liquid contaminated the core even more than the Vostok liquid
(Boutron et al 1987). Pollutants enter deep into the core through
the micro-fractures, even in parts at the depth at which no visible
cracks were reported (Boutron et al 1987).
Several papers provide evidence for serious contamination of
deep Antarctic cores with metals and major ions (see below). The
factors that caused this contamination also influenced the gas
composition of the cores. The contamination occurred along the
natural capillary netowrk of liquid veins, and along a network
of micro- and macro-cracks.
MACRO- AND MICRO-CRACKS
The cracks in the ice are first formed by sheeting in the bottom
of "dry" and "wet" boreholes due to partial
release of the load pressure; then due to vibration or thermal
shock during drilling and later due to pressure relaxation of
the cores. Macro- and micro-cracks occur in ice cores from all
depths below about 100 m. The volume of micro-cracks in deep ice
cores reaches atoub 10% of the volume of gas inclusions (Gow 1971).
It was reported that large parts of the Vostok core were "badly
fractured" (De Angelis et al 1987), and that "the upper
part of the core (above 1000 m) shows many cracks... which may
have been contaminated by drilling fluid" (Petit et al, 1990).
Between 125 and 700 m depth the core was extremely fractured (Legrand
et al 1988). This is the region where dramatic decreases in CO2
concentrations, and changes in stable isotope ratios, were observed.
In the classical papers on CO2 and stable isotopes in the Vostok
core, the possibility that this decrease and these changes may
be related to post-drilling effects were not considered. To prevent
the breaking of ice due to fracturing, the temperature of the
Vostok core sections was increased in the laboratory to -3ºC
for 24 hrs before analysis (Legrand et al 1991). Such treatment
of the cores must lead to redistribution of their chemical components
due to thermal gradients (Jaworowski et al 1990b) and the increase
in volume of the liquid phase.
Deep drilling in ice sheets involves extremely brutal treatment
of the ice samples. This procedure leads to dramatic changes in
the recovered cores. We will describe here such changes using
the Vostok core as an example, and the technical data on thermal
drilling from Kudryashov et al (1984a,b,c) and Zotikov (1986).
The thermal drills at the Vostok station operated at working
temperatures of -60 to +120ºC. The power flux per unit area
of the heaters was about 20 W/cm^2, and the power consumption
was 4.5 - 6 kW per heater. The body of the heaters was made of
copper and aluminium. The speed of drilling was about 8.5 cm/min,
and during 1 min about 700 ml of ice was melted. The meltwater
was mixed with a rapid stream of a hydrophobic drilling fluid.
The fluid was forced by a special device through the openings
in the heater to the bottom of the borehole, and pumped out via
heated tubes operated at 350 W of power per running meter length,
ie a total of 600 kW for the 2000 m borehole, to avoid refreezing
of the meltwater.
Due to these operational conditions, with the heater at a temperature
close to the boiling point of water, the hot mixture of meltwater
with the hightly contaminated drilling fluid penetrated the core
via macro- and micro-cracks. This penetration occurred at the
moment of passing the heater through the bottom ice, and then
in the "inner tube" of the drill, filled with the drilling
fluid. Some of its components (eg ethylene glycol monoethl ether)
are excellent solvents for metals, and dissolve aluminium and
copper from the heaters during operation at high temperatures.
The total amount of drilling fluid in the 2000-m deep Vostok
borehole was about 28 metric tons. The drilling fluid was used
to counter the load pressure on the borehole. This, however, was
never achieved completely, because the fluid level was maintained
200-300 m below the surface of the ice sheet. The difference between
the cryostatic pressure and the drilling fluid pressure in the
Vostok borehold, at any depth below 200 or 300 m, can be calculated
from the densities of firn and ice between the surface and a depth
of 300 m (given by Barkov et al 1975; and Korotkevich et al, 1978),
and from the average density, 0.924 g/cm^3, of the drilling fluid
(Kudryashov et al 1984a). At a drilling fluid level of 200-300
m depth this pressure difference will be between about 15 and
24 bars, respectively.
Such a pressure difference is sufficient to cause horizontal
sheeting of the ice in the bottom of the borehole, before drilling
a new portion of the core. Thus the drilling head penetrates ice
which is already fractured or preconditioned for fracturing. Cracking
is also likely to be induced by thermal shock, caused by a rapidly
formed steep thermal gradient, which may reach about 150ºC
across the 1.5-cm thick ice between the surface of the heater
and the ice core interior ahead of the drill (Zotikov 1986).
Sheeting is a well-known phenomenon in geology, and usually designates
horizontal fracturing due to stress-releases within a rock mass.
The fractures occur approximately perpendicular to the volume
extension. Sheeting is caused by elastic relaxation of compression,
which proceeds rapidly in ice. After just a few minutes, this
accounts for about a 0.2% volume increase of ice cores (Gow 1971).
Volume changes of the order of tenths of a percent may form such
stratified sheeting cracks (Hobbs et al 1976). Sheeting also occurs
in relaxed ice cores (Jones and Johari 1977), in which the volume
increase may reach 0.6%, and the ice density decreases by 2-9.5
kg/m^3 (Nakawo 1986) or more. Typical horizontal sheeting "stratifications"
were observed in cores, starting at a depth just below 100 m,
where the pressure relaxation was about 8 bars.
Narita (1978) presented photographic evidence of dense horizontal
stratification comprised of thin layers in ice cores from the
Mizuho station (East Antarctica) drilled thermally without a hole-retaining
fluid. These layers appeared below a depth of 108.25 m and were
observed down to the bottom of the core at 145.35 m. A similar
"stratification" due to cracks was observed by Nakawo
and Narita (1985) in samples from a 413.5 m deep Mizuho core,
1-2 days after core recovery. After 2 days storage the cracks
were partly healed. Micro-cracks were revelaed by the microscope
in a thin piece of specimen taken from in between two macro-cracks.
This fracturing is presented schematically in Figure 6.
Nakawo and Narita (1985) found that, below a depth of 110 m,
where the samples were rich in cracks, the bubble pressure was
much lower than the load pressure. This difference was due to
gas release through the cracks. A similar pressure reduction was
recorded in ice cores from various sites (Langway 1958; Gow 1968b;
Gow and Williamson 1975). In the Mizuho core the cracks were associated
with up to a ~40% decrease in the gas content of the ice (Nakawo
and Narita 1985). For the same Mizuho core, Narita and Nakawo
(1985) reported pressure-cracked bubbles, with "brims"
which might be caused by thermal shock and pressure release. Similar
crack rings, and a fissure-like distortion of the ice structure,
were observed in and around relaxed air bubbles by Shoji and Langway
Narita and Nakawo (1985) observed a substantial difference in
the ice structure of cores examined within 1 month, and 1 year
after recovery. The most important textural change was in the
crystal size, increasing with time, due to recrystallization.
They found that this effect was quite large, even in shallow cores
from a depth of <100 m. An important implication of this finding
is that the observations on crystal size in ice cores stored for
several years before analysis represent post-drilling processes
rather than the temporal distribution of crystal size in the ice
sheets, which was interpreted as being related to climatic changes
(Duval and Lorius 1980; Petit et al 1987).
One of us (Z.J.) observed horizontal stratification in Mizuho
cores from 105.97, 356.03 and 657.69 m depth, but not at a depth
of 79.39 m. The observation was carried out 7-19 years after collection
of the cores, which were stored at -20ºC. The horizontal
strata were densely distributed throughout the whole length of
the cores. The cores revealed no macroscopic cracks when inspected
on a light-table. The strata appeared at about 2-5 mm intervals
and were associated with delicate horizontal corrugations of the
surface. The rugae reached a depth of 0.5 - 1 mm. The tint of
the striations was different from that of the enclosing ice. When
successive concentric 1 cm thick veneers of ice were removed from
each of these cores, the deformations produced at the surface
were evident throughout the whole cross-section of the cores (Fig
7). This horizontal stratification is a remnant of old cracks,
completely healed by regelation. Such cracks, before they were
healed, are shown schematically in Figure 6. The same remnants
of the old cracks are clearly visible in a photograph of the Vostok
ice core published by Schneider (1989).
The visible and invisible cracks, both in the shallow and deep
cores, are an important factor influencing the CO2 content and
isotopic composition of gases recovered from the gas inclusions.
In a clasic paper on the CO2 content of the Vostok core there
is no information given on the method of drilling, and on its
eventual effects on the state of the ice core (Barnola et al 1987).
The reader is not informed that the core was thermally drilled
with a hydrocarbon liquid used in the borehold as a wall-retaining
fluid (Kudryashov et al 1984b). This "drilling fluid"
consisted of a mixture of a regular military airplane fuel "TS-1"
and a "loading material" composed of trichloroethylene,
tetrachloroethylene and "Khladon-II" added for density
regulation and antifreeze purposes (Kudryashov et al 1984a). According
to Gielo-Klepacz (1991), "Khladon-II" is the ethyl ether
of ethylene glycol (CH3--CH2--O--CH2--CH2--OH), heavily contaminated
with Zn (dissolved from containers), and "TS-1" fuel
is contaminated with resins, sulphates, inorganic acids, alkalis
(NaOH and KHO), traces of metals and suspended particulates. About
0.3 mg KOH per 100 ml is usually needed to neutralize the acidity
of the "TS-1" fuel.
As may be inferred from Koudelka (1964a,b), due to the highly
increased solubility of CO2 in mixtures of organic solvents, the
drilling fluid is probably enriched in CO2 from the atmosphere.
This fluid, mixed with meltwater, transferred external pollutants
into the Vostok core.
CONTAMINATED ICE CORES
The traces of hydrocarbon drilling fluids contained in the macro-
and micro-cracks of the ice cores may be counted as CO2 in the
gas chromatograph in which a flame ionization detector converts
CO2 to methane (Stauffer et al 1981). A proportion of the hydrocarbons
entering the sample chamber will be converted to methane by cracking.
This methane will mix with the methane formed from the CO2 and
give apparently higher readings.
The solubility of CO2 in mixtures of water and organic solvents
present in the drilling fluid is even higher than in the pure
solvents (Koudelka 1964a,b). Therefore one may expect both decreases
and increases of CO2 in air recovered from the ice cores, depending
on the spatial patterns and history of contamination with drilling
Some of the cracks in the Vostok core, even when annealed, were
identified because they contained traces of drilling fluid (De
Angelis et al 1987). These authors tried to decontaminate the
core, first by an ultrasonic bath with acetone at -15ºC,
and then by rinsing the samples with ultrapure water "until
all fractures were removed". Legrand et al (1988, 1991) reported
that 50-90% (sic!) of the ice core were eliminated during a "cleaning"
procedure in which the temperature of the water was about 20ºC.
As indicated by field and laboratory observations, such a procedure
must lead to a dramatic redistribution and removal of the bulk
of impurities from the ice samples with the meltwater. Up to 80%
of H+, nitrates, sulphates and heavy metals may be contained in
the first 30% fraction of water releasing some ions more quickly
than others (Gjessing et al 1976, 1991; Gjessing 1977; Johannessen
et al 1977; Davies et al 1982), and will lead to dramatic changes
in the stable isotope ratios of the remaining ice (Segalstad and
Jaworowski, in preparation).
The analytical results from such cores, subjected first to contamination
by highly polluted drilling fluids and then to such cleaning procedures,
were often used for paleo-climatological interpretations (eg Legrand
et al 1988; Petit et al 1990). However, they cannot be regarded
as representing the original chemical composition of the ice.
An indication of the magnitude of drilling effects on the chemical
composition of ice cores is the finding that Pb and Zn concentrations
in the Vostok core decreased by a factor of about 10,000 and 400,000
respectively, between the surface and the centre of the core (Boutron
et al 1987, 1990). The concentrations of Pb and Zn in surface
East Antarctic snow are similar and low (Table 3). But in the
Vostok core the concentrations of Pb were found to be one to two
orders of magnitude lower than the concentrations of Zn, and were
1000 and10,000 times higher than in surface snow. In the core
sections from about 2026 m depth the concentrations of Pb were
found to be 31,400 pg/g, and those of Zn 300,000 pg/g. Even ~3
cm inward from the surface of the Vostok core the Pb content was
more than 1000 times above the level at its centre (Boutron et
al 1987). In some Vostok core sections the Pb concentrations were
decreasing towards the centre of the core without showing plateaus
(Boutron et al 1988). According to Boutron et al (1988) "contamination
has penetrated to the very centre of the Vostok core either during
drilling, transport and storage and/or during mechanical decontamination
in the laboratory". Other cores from shallower depths, recovered
without drilling fluid, were less contaminated with Pb and Zn
than the Vostok core (Table 3).
Non-heavy-metal pollutants also contaminated the Vostok core.
Even in its central parts the concentrations of SO4^2-, Na, and
Al were one to two orders of magnitude higher than in the surface
snow, as seen in Table 4.
The highest contamination of the inner parts of the Vostok core
with Pb, Zn, Al and Na were reported by Boutron et al (1987, 1990)
for a depth around 500 m, from where extensive cracking was reported
(Legrand et al 1988); and for a depth near 2000 m, where no visible
fractures were found (Boutron et al 1987), but where micro-cracks
are larger and more numerous (Gow 1971). This indicates that contaminants
penetrated via the micro-fractures or the natural capillary network
of liquid at the moment of drilling or soon after, ie during pulling
of the core and before the fractures were "healed".
The high Al content at these two depths of the Vostok core were
regarded by De Angelis et al (1987), Petit et al (1990) and Jouzel
et al (1990) as a dust index (they assumed that Al represents
7% of dust composition), and interpreted as indicating climatic
changes. In the light of the foregoing discussions such an interpretation
does not hold, as it is obvious that the Al content was due to
contamination of the cores. Dramatic decreases in CO2 content
were reported to occur at the same two depth regions of the Vostok
Such a distribution of metal concentrations in the cores is obviously
caused by contamination from drilling fluid and mechanical devices.
However, radial redistribution of impurities in ice cores may
also occur due to thermal gradients developing during drilling,
transportation, storage, and handling of the cores in the laboratory.
During thermal drilling at the Vostok station, the original temperature
of the ice in the core ranged between -35 and -57ºC, but
the core surface was exposed to a temperature near +100ºC
(Kudryashov et al 1984a) and then cooled again to about -57ºC.
Post-drilling operations may increase the temperature of cores
close to 0ºC or more (Boutron and Patterson 1983; Pearman
et al 1986). Thus the total thermal gradients in the Vostok core
may reach up to about 150ºC!
Redistribution of major ions, acids and several inorgainc radioactive
tracers was observed in firn and ice samples exposed to a thermal
gradient of -21 to -1ºC (Jaworowski et al 1990b). Also, Satow
and Watanabe (1985) observed substantial changes in the isotopic
composition of oxygen in a block of Antarctic firn after it was
exposed to a small thermal gradient in the laboratory.
This discussion indicates that attempts to find the "true"
concentration of gases, metals, or other substances in deep ice
cores, must be regarded as a very difficult and rather unreliable
exercise. The reason is that all deep ice cores have been exposed
to high contamination, thermal gradients and structural disturbances.
DISCREPANCIES IN DEEP
The CO2 concentrations found in air bubbles and in secondary air
cavities of deep Vostok and Byrd cores range between 178 and 296
ppm ie much below the current atmospheric level. These values
were interpreted as representing the real composition of the ancient
atmosphere and past climatic changes. This would mean that the
current level of the gas, about 350 ppm, is a new and unique phenomenon,
and that during the past 160,000 years the atmospheric C02 level
never approached the current value, not even some 65,000 and 110,000
years ago when the global sea-surface temperature was 2-3º
C higher than now (Shackleton and Opdyke 1973; Ruddiman 1985;
Ruddiman and Raymo 1988), or 6000 years ago when the air temperature
between 60 and 75ºN was 4.5ºC higher than now (Zubakov
and Borzehkova 1990). If this low ancient C02 atmospheric level
were true, it would mean that this gas had no influence on past
climatic warmings, and that these warmings did not increase its
atmospheric level by, for example, degassing from a warmer ocean.
There are numerous contradictions between the isotopic and CO2
records from deep Antarctic and Greenland ice cores. For example,
glacier isotopic data have been taken to indicate that the present
warmer period occurred in Antarctica several thousand yers earlier
than in the Northern Hemisphere (Radok 1985). This is obviously
not a realistic estimate. The rapid variations of CO2 concentrations
of 50-70 ppm, observed by Stauffer et al (1984) in Dye 3 and Camp
Century (Greenland) ice cores from a period 30,000 to 40,000 years
BP, are not similar to the much smaller variations in the Antarctic
Byrd cores (Neftel et al 1988; Oeschger et al 1988) or Vostok
cores (Barnola et al 1987).
13C/12C ratio in the carbon dissolved in the deep ocean was lower
during the glacial than interglacial times, suggesting higher
atmospheric CO2 content during the glacial time (Shackleton 1977).
This is contrary to the ice core data indicating that the atmospheric
level of CO2 was substantially lower in the glacial period than
today. To increase the CO2 content from 200 ppm in the glacial
period to 275 ppm in the interglacial by warming the surface ocean
would require a 10ºC increase in sea surface temperature.
However the faunal record in ocean sediments indicate that the
interglacial sea surface temperature warming was only 1.7ºC
(Broecker and Peng 1986). We agree with Broecker and Peng's statement
that "one way out of this dilemma would be to challenge the
validity of the ice-core-based glacial to interglacial atmospheric
The reduction in the atmospheric concentration of CO2 during
the last glaciation, deduced from ice core measurements, was explained
by more efficient "biological pumping" of carbon into
deep oceanic waters, due to increased bioproductivity of the ocean
(Martin 1990). However, records of the accumulation rates of diatom
shells, the ration of germanium to silicon in diatomaceous opal,
and the carbon isotope ration in foraminiferal carbonate indicate
lower productivity during the last glaciation (Mortlock et al
1991). According to Barnola et al (1987) the levels of CO2 in
the global atmosphere during many tens of thousands of years spanning
30,000 to 110,000 BP were below 200 ppm. If this were true then
the growth of C3 plants should be limited at the global scale
because their net photosynthesis is depressed as the CO2 concentration
in air decreases to less than ~250 microbar (less than ~250 ppmv)
(McKay et al 1991). This would lead to the extinction of C3 plant
species. This has, however, not been recorded by paleobotanists
Stauffer et al (1984) noticed a serious discrepancy in their
own analytical results from Dye 3 (Greenland): a lack of lag between
their CO2 and delta-18O data which should be observed if the results
were representing the ancient atmosphere.
According to Fairbanks (1990) the assertion based on the delta-18O
anomaly in the Dye 3 (Greenland) ice core that the climate during
the Younger Dryas (10,000 to 11,000 years BP) rapidly shifted
(within less than a century) into and out of "glacial conditions"
has been repeated so often that many believe it has been firmly
established. This author, however, found from corrected 14C data
that the end of the delta-18O anomaly in Dye 3 is neither correlative
with the end of the Younger Dryas climatic event identified in
pollen records as marking vegetation changes is Europe, nor with
its dating based on marine faunal indices. Also, data from the
Byrd core (Antarctica) do not show the rapid delta-18O changes
indicating warm periods (Oeschger et al 1988). Robin (1985) pointed
out that the paleotemperature changes reconstructed from the Vostok
ice dated 106,000 to 116,000 BP do not agree with other ice, marine
and terrestrial data.
Perhaps even more serious is the discrepancy in CO2 data for
Antarctic cores from the Byrd and Vostok stations. The first important
decrease in CO2 concentrations occurred in the Byrd core between
about 500 and 600 m depth (Neftel et al 1982), and in the Vostok
core at a similar depth of about 400 m. But the assigned age of
the ice in both these cores is totally different: 1500-1600 years
BP in the Byrd core, and 18,000 years BP in the Vostok core. The
second CO2 decrease, of about 50 ppm, occurred in the Byrd core
in a region between 950 and 1200 m depth, where the age of the
ice was about 8000 to 14,000 years BP. In the Vostok ice of this
age (at about 250-300 m depth), CO2 concentrations were rather
stable, and in the ice deposited 12,000 to 16,000 years ago were
higher by about 60 ppm than in the Byrd core. On the other hand,
at a depth of about 1100-1150 m the CO2 concentrations were similar
in the Byrd and Vostok cores (about 235 ppm), although the assigned
age of the ice differed by about 65,000 years.
An ad hoc attempt to explain some of these discrepancies by Oeschger
et al (1988) by: (1) "a process which has not yet been identified",
(2) wrong modelling, and (3) "not overlapping time intervals,
perhaps due to rheological irregularities" is ambiguous and
All these similarities and differences are rather fortuitous,
as they are due to artifacts in the ice sheets and in the ice
cores. In some cases these artifacts have a random distribution,
but they may also depend on processes related to depth in the
ice sheet, and therefore show some regularities.
AGE OF AIR IN ICE
The validity of the hypothesis that the age of the occluded air
is younger than that of the ice matrix is of crucial importance
for the estimates of CO2 levels in the pre-industrial atmosphere.
This hypothesis was proposed by Berner et al (1980) who assumed
that for Greenland the typical age difference is 200 years. The
hypothesis was then accepted by Craig and Chou (1982) who assumed
that the air trapped in ice at 68 m depth at Dye 3 (Greenland)
is 90 years younger than the age of the ice. Schwander and Stauffer
(1984) elaborated the hypothesis and estimated the age difference
for eight Greenland and Antarctic sites ranging from 90 to 2800
years. It should be noted that the hypothesis is purely speculative,
neither based on experimental nor empiric evidence, and has an
ad hoc flavour.
The hypothesis was based on the assumption that the air in the
porous firn strata is well mixed with the atmosphere at least
down to the beginning of the firn-ice transition. Furthermore,
is was assumed that air trapped in the ice sheets is isolated
from the atmosphere only after a complete enclosure of air bubbles,
at an ice density of 0.83 g/cm^3. At the Antarctic ice sheet this
density is reached between about 50 and 100 m depth (Gow 1968a;
Watanabe et al 1978; Barnola et al 1987). To satisfy this assumption
the condition is needed that the whole firn column, down to the
density level of 0.83 g/cm^3, is devoid of layers impermeable
to air. It was believed that this condition can be met at the
cold Antarctic sites with mean annual temperatures of about -24ºC,
where, as was assumed, no summer melting occurs. This supposition
is not true, as radiational melting was observed in Antarctica
even at locations with summer temperatures near -20ºC (see
section Liquid in Ice), and the existence of thick melt layers
and thin ice crusts was reported from many Antarctic localities
(the Siple, Byrd, Mizuho and Vostok stations) with mean annual
temperatures ranging from -24 to about -57ºC.
At the Siple station (mean annual temperature -24ºC), Neftel
et al (1985) reported the existence of one 10-mm thick melt layer
at a depth of 7 m. But from the Siple Coast (mean annual temperature
-26.5ºC) Alley and Bentley (1988) reported nine melt layers,
some of them more than 20 mm thick, distributed randomly in a
100 m deep core. At the Byrd station, hundreds of thin ice layers
were reported from several cores by Gow (1968a). At this site,
layers of bubble-free ice, up to 1 mm thick, were reported to
occur in the firn with a frequency ranging from one to eight layers
per metre. These ice crusts could be identified in cores from
as deep as 240 m (Gow 1968a). Several refrozen melt layers up
to 100 mm thick were observed in the Byrd core at between 1300
and 1700 m depth (Gow 1970).
One of the proponents of this hypothesis stated that "the
ice layers do not cover a large region continuously but are more
or less frequently interrupted by holes or dislocations. According
to my knowledge, there exist no extensive investigations on the
geometry of ice layers" (Schwander 1989). But meticulous
Japanese studies, covering a vast Antarctic region, indicate the
At the Mizuho station, Watanabe et al (1978) observed ice layers
with a frequency of about 5 to 15 per metre down to a depth of
110 m. A stratigraphic study of several metres thick upper firn
strata at 38 sites in Enderby Land (Northeastern Antarctica) was
carried out by Watanabe (1977) along a traverse reaching about
1200 km inland and an altitude up to 3382 m, where the average
annual air temperature is -48ºC. This study demonstrated
that ice layers are ubiquitous within the Antarctic ice sheet.
They form a complex multilayer structure in which merging neighbouring
layers separate firn in horizontal pockets (Fig 8). Watanabe (1977)
found that the ice layers maintain their continuity along the
whole studied cross-section of firn, and that they continue horizontally
for at least several tens of kilometers (Watanabe 1978).
In a 6 m deep snow and firn layer at the Vostok station, eight
sun ice crusts and 19 wind crusts were found by Kotlyakov (1961).
A well-defined stratification was also observed in the Vostok
core down to several tens of metres below the surface (Korotkevich
et al 1978).
In the top 200 cm of firn at a site at Dronning Maud Land (Antarctica),
where the mean annual temperature is about -19.2
ºC (Orheim et al 1986), Repp (1978) found several 1-2 cm
thick ice layers, some of them clustered together to a total thickness
of 6 cm.
In very cold regions, thin sun ice crusts, about 1 mm thick,
form near the surface of the firn due to insolation and refreezing
of water vapour, evaporated or sublimated from ice crystals below.
The vapour migrates upwards along the decreasing temperature gradient,
and condenses in the interstices between grains of cold snow at
the surface or below a less permeable glaze or sublimation crust.
These processes are discussed in detail by Gow (1968a) and illustrated
in Figure 9. The changes in the state of H2O in these processes
have an important impact on the stable isotope, elemental and
molecular composition of ice and of air entrapped in it.
The load pressure in the firn forces the air up through the leaks
in the thin ice layers. This permanent upward stream of air prevents
atmospheric air from penetrating downwards through the laminated
structure of firn.
As proof of the "age of air" hypothesis, Schwander
and Stauffer (1984) cited measurements of radioactive atmospheric
trace gases in polar ice cores of Loosli (1983), and stated that
"39Ar measurements provide evidence that the air in the open
pore volume is well mixed at least down to the beginning of the
firn-ice transition". However, the results in Loosli's (1983)
paper indicate the opposite. At 70 m depth at Dye 3 (Greenland),
the age of natural racioactive 38Ar gas trapped in air bubbles
was about 70 years, ie the same as the age of the ice determined
by other methods. Also, at a depth of about 100 m at the Byrd
station (Antarctica) the age of the air measured by the 39Ar method
was the same as the age of the ice estimated from the precipitation
rate (Oeschger et al 1977). However, all these argon age determinations
were highly uncertain, because Loosli (1983) found that all ice
samples were contaminated by ambient air, and even in the 3000
years old ice of high density, the total measured 39Ar activity
has to be attributed to contamination by atmospheric air during
field and laboratory processing. By measuring 85Kr, a radioactive
gas produced in nuclear reactors and bomb explosions, Loosli (1983)
demonstrated that up to 36% ambient air was found in gas samples
extracted from ice cores. These cores were stored for about 8-13
years before analysis, and exposed to the ambient atmosphere.
This finding has an important implication. It indicates that
unsealed ice cores, such as those used for CO2 and stable isotope
measurements, are contaminated by ambient air. Some of these cores
were even exposed to cigarette smoke (Jaworowski et al 1990a),
which indicates that contamination problems are not seriously
considered in gas analysis of ice cores.
Discussing the age of entrapped air, Schwander (1989) did not
quote Loosli (1983), and stated instead, incorrectly, that "at
present no data from [radioactive atmospheric] tracer methods
are yet available". He also, rightly, did not quote the circular
argument of Neftel et al (1985) that the same concentrations of
CO2 found in the recent South Pole atmosphere and in air bubbles
trapped in nineteenth century Antarctic ice prove that air from
bubbles is 82 years younger than the ice. This finding suggests
rather that the atmospheric CO2 concentration 82 years ago was
the same as now.
Craig et al (1988a) revoked their earlier assumption that trapped
air at Dye 3 is 90 years younger than the ice in which it is sealed
(Craig and Chou 1982). They stated that results of their isotopic
measurements of methane recovered from the Greenland ice "preclude
such a large difference in age".
Craig et al (1988b) observed enrichments of 15N and 18O isotopes
and of N2 and O2 in air trapped in polar firn strata effects resulting
from molecular diffusion and other processes. This strongly indicates
that the air in porous firn is protected from convective and other
The above discussion demonstrates that the hypothesis that entrapped
air is much younger than the ice enclosing it disagrees with glaciological
evidence. In polar ice sheets, the air in firn is isolated from
the atmosphere by several hundred thin ice layers, before the
firn pores close off completely at the firn-ice transition. This
means that the age of the gas in a primary air bubble is approximately
the same as the age of the ice that encloses it.
This hypothesis was posed at the time when the CO2 concentration
in air bubbles from ice deposited in the nineteenth century or
earlier was found to be similar to the current atmospheric level.
The ad hoc hypothesis was needed to make this finding consistent
with the man-made global warming paradigm. This is shown in Figure
10a, which illustrates that the CO2 concentrations in pre-industrial
ice from Antarctica were made to overlay exactly the recent atmospheric
concentrations at Mauna Loa (Hawaii) (Siegenthaler and Oeschger
Figure 10a was prepared in connection with "model calculations
that are based on the assumption that the atmospheric [CO2] increase
is due only to fossil CO2 input" and other human activities.
The curve in this figure is gased on data of Neftel et al (1985)
and Friedli et al (1986) from the Siple ice core, which was partially
melted during transportation. The overlaying was done after assuming
that the age of the air is 95 years younger than the ice. However,
as shown in Figure 10b, without this speculative assumption the
Mauna Loa and Siple data do not overlay. Rather than representing
past atmospheric changes, the ice CO2 concentrations in the Siple
core decrease with increasing load pressure up to about 15 bars,
due to clathrate formation, differential dissolution of gases
in the liquid water at the surface of ice crystals, and other
processes in the ice sheet and in the ice core. As discussed above,
studies at other locations, carried out before 1985, recorded
higher CO2 levels in air from pre-industrial ice than in that
from both the Siple core and at Mauna Loa.
Figure 10a was reproduced in countless publications and used
as proof that the burning of fossil fuels, deforestation and changing
land use by man have changed the global carbon cycle very significantly
during the last two centuried. The figure was used as the basis
for other speculative estimates of changes in the global carbon
cycle (eg Bolin et al 1989). It is in this context that one should
not that Figure 10a does not represent real CO2 changes in the
global atmosphere, but rather processes in the ice sheets, artifcacts
in the partially melted ice core, and unsubstantiated assumptions
on the age of air inclusions. It is astonishing that such non-representative
and misleading data have been so widely and uncritically accepted.
CHANGES IN THE CHEMICAL
COMPOSITION OF TRAPPED AIR
The trapping of CO2 in glaciers starts in the atmosphere when
this gas associates with snowflakes by dissolution in water droplets
prior to freezing, and in a liquid present at the surface of the
flakes, and by adsorption onto crystal surfaces. A liquid containing
sulphuric acid is incorporated into snow crystals as a film on
a solid nucleus or added by riming, and it is expected to remain
as a liquid on the outside of the ice grain crystals (Wolff et
al 1988). About 2-3% (by weight) of supercooled liquid water may
be mixed within the entire mass of snowflakes (Matsuo and Miyake
1966). The mechanisms of in-cloud and below-cloud scavenging of
gases by supercooled water were reviewed by Davidson (1989).
The presence of liquid water in polar ice sheets is supposedly
the main factor involved in suppressing the seasonal acidity peaks
in Pleistocene ice (Hammer et al 1985), and in rapid smoothing
in the upper strata and obliteration of deltaD and delta18O short-term
variations in ice older than 10,000 years (eg Langway 1967; Dansgaard
1977; Johnsen 1977; Grootes et al 1990).
Dissolution and adsorption of CO2 continues after deposition
of snow on the glacier. Due to large solubility differences of
particular air gases, the composition of air trapped within snow
changes at this early stage and during firnification. At high
pressures deep in the ice sheets, the equilibrium pressures of
gases dissolved in water increase at different rates, and are
higher for CO2 than for other air gases (Enns et al 1965). Therefore,
one may expect that the composition of the air trapped in firn
and ice will be different from that in the atmosphere: the air
bubbles will be depleted in CO2; and the O2/N2 and Ar/N2 rations
will be changed in the whole ice and in the air bubbles.
Several in situ processes in the ice sheets and in the cores
cause changes in the composition of entrapped gases. Both types
of processes depend on pressure and temperature varying with depth
in the ice sheets. Therefore, on may expect that these changes
should reveal some regularities at a greater vertical or temporal
scale. At a smaller vertical scale, or the order of a few metres
or centimeters, and also at the same horizons, one should expect
a random distribution of these changes.
These processes usually lead to a low CO2 content of voids in
the firn and bubbles in the ice. They were also reflected in large
local fluctuations in CO2 concentrations in air bubbles collected
from a 30 cm long ice core from Dome C (Antarctica), where annual
precipitation is 3.6 cm water equivalent. The precision of determinations
was 3%. The ice samples (1-2 g) were taken from 1.5 cm thick slices
of core, ie about two slices per year of ice accumulation. The
concentration of CO2 fluctuated by more than 13% (32 ppm) between
neighbouring slices, and by about 10% (25 ppm) in the same slice
(Barnola et al 1983). As mentioned before, these fluctuations
are twice as high as the "CO2 glacier signal" used in
support of claims of anthropogenic increase of this gas, and they
certainly do not reflect the atmospheric fluctuations in the same
Oeschger et al (1985) stated that various processes could lead
to insignificant CO2 enrichments in ice of not more than 10 ppm,
and that "the analysis of the N2/O2/Ar ratios in ice originating
from very cold areas with no summer melting shows that, within
experimental uncertainty, the measured ratios agree with those
in air" (Raynaud and Delmas 1977). But Raynaud and Delmas
(1977) interpreted their analytical results from the Pionerskaya
and Vostok stations differently: "par rapport a la composition
atmospherique... les effets observes sont donc faibles mais significatifs
et indiquent un appauvrissement relatif des teneurs en O2 et Ar".
In the interstitial air pumped from the upper firn layer at these
two stations, Raynaud and Delmas (1977) found negative enrichment,
corresponding to delta O2/N2, and delta Ar/N2 values of -5.3 and
-0.11%o [permil] in comparison with the free atmosphere. As mentioned
before, they also found CO2 depletion of about 90 ppm in interstitial
air from recent firn at Vostok and 170 ppm at Pionierskaya.
Large negative and positive enrichments of heavy gases' (O2 and
Ar) ratios to N2 were reported by Craig et al (1988b) in air recovered
from Antarctic and Greenland cores. The values for deltaO2/N2
ranged from -14 to +3.5%o [permil] and those for delta Ar/N2 from
-7 to +4.2%o [permil]. The authors suggested that differential
capillary flow during the loss of gas from core samples via micro-cracks
is responsible for this effect.
In the early studies it was found that the concentration of O2
in ice from Greenland and Antarctica decreases with depth, and
that the concentration of N2 increases (Lorius et al 1968; Alder
et al 1969). This may be due to gravitational separation of heavier
and lighter gas molecules (O2, mol. wt. =31.9988; N2, mol. wt.
= 28.0134) in the porous firn.
Also, Horibe et al (1985) found negative enrichment for delta
O2/N2 and deltaAr/N2 in an ice core from Camp Century (Greenland).
These authors reported depletion of oxygen by up to 1.5% in air
bubbles trapped in the 20,000 years old ice, in comparison with
the ambient atmosphere; the depletion increased with depth in
the ice sheet between 559 and 1248 m.
The results of measurements of N2, O2, Ar and CO2 trapped in
ice, compiled in Table 5, indicate that the composition of air
in the whole ice and in the air bubbles differs greatly from that
in the atmosphere, both in temperate glaciers and in polar ice
LIQUID IN ICE
Faraday (1850) postulated that a thin water layer exists on ice
surfaces at temperatures below the melting point. Later it was
found that snow and ice consist of crystals, separated by thin
films of intercrystalline brine, which contains soluble impurities
(Buchanan 1908). But it was only recently recognized that this
phenomenon also occurs at very low temperatures.
Large numbers of under-ice puddles of meltwater were found in
Antarctica at a depth of 10-20 cm below the surface of sea ice.
The puddles, up to 70 cm deep, were formed at a mean air temperature
of -2ºC, and the temperature of meltwater under the ice cover
in the puddles reached up to +10ºC. The mode of formation
of the puddles suggested that the cause of their formation was
sun radiation absorbed in the interior of the ice (Endo 1970).
In Dronning Maud's Land (Antarctica) a firn layer, soaked with
liquid water, was observed at a site where the average annual
temperature is about -19ºC (Repp 1978). Large amounts of
meltwater were also found in ponds at a depth of about 1 m under
the ice surface in Dronning Maud Land, at an altitude of about
1200 m, where the summer air temperature was below -20
ºC (Hagen 1991). This phenomenon may contribute to the formation
of impermeable layers in the firn in very cold regions of Antarctica
and to the redistribution of CO2.
Field observations have shown the existence of unfrozen water
in porous media at temperatures between -30 and -50ºC (Anderson
and Morgenstern 1973). This has been attributed to various mechanisms,
including diploar forces, density variations and pressure melting
of ice (Dash 1989).
Thermodynamic models suggested that a layer of a definite thickness,
with properties similar to those of the surface of liquid water,
exists at the surface of ice crystals to a temperature of -15ºC
(Fletcher 1973). Nye and Frank (1973) proposed that liquid water
in ice is located at the triple junctions between the crystals
which form an interconnecting network in the ice. The presence
of a quasi-liquid layer on the surface of ice crystals at a temperature
of about -35ºC was postulated by Orem and Adamson (1969)
on the basis of N2 adsorption studies. Alsok Ocampo and Klinger
(1982) deduced from CO2 adsorption onto ice that the ice surface
is covered by a quasi-liquid layer at temperatures above -35ºC.
The existence of a quasi-liquid in ice was detected by proton
magnetic resonance spectroscopy. With this method Kvilidze et
al (1970), Bell et al (1971) and Kvilidze et al (1974) demonstrated
that liquid water is present in ice down to temperatures of about
-13ºC. Photoemission studies with ultraviolet radiation (Nason
and Fletcher 1975) provided evidence for the presence of a water-like
layer on the surface of ice down to -160ºC. This layer becomes
progressively thinner as the temperature decreases. The thickness
of this layer was estimated to range between a few nanometers
and a few tens of nanometers (Golecki and Jaccard 1978). We postulate
that such a layer covers the inner surfaces of air bubbles in
ice sheets and the secondary gas cavities in ice cores, and that
it contibutes to the depletion of CO2 from the gas inclusions
and to isotopic equilibration of gases with the ice.
This is in agreement with the observed oxygen isotope equilibration
between CO2 gas inclusions in ice and H2O observed by Siegenthaler
et al (1988). These authors asked the question whether this could
be due to the existence of a liquid-like layer at the air-bubble-ice
Maccagnan (1981) and Maccagnan and Duval (1982) proposed, on
the basis of direct current conductivity measurements, that the
liquid in polar glacier ice is a water-sulphuric/nitric acid mixture,
located at the ice grain boundaries. A model study by Wolff and
Paren (1984) suggested that these mixtures are located at triple
junctions between ice crystals. This vein system may provide a
means of draining liquids out of ice sheets.
Liquid sulphuric acid in deeply frozen glacier ice was discovered
by Mulvaney et al (1988). By scanning electron microscopy and
X-ray spectra they found that high concentrations of sulphuric
acid are located at the triple junctions, but they could not rule
out the presence of some liquid in films at two-grain boundaries.
This study, performed on 125-year-old ice from the Antarctic Peninsula,
indicates that the concentration of sulphuric acid in the veins
may range between 2.5 and 4.9 M. At this higher concentration
the acid-water mixture may remain liquid down to the eutectic
temperature of -73ºC. Sulphuric and nitric acids are common
impurities in polar ice sheets. Studies by Wolff and Paren (1984)
and Mulvaney (1988) indicate that most of these impurities are
eliminated from the ice crystal lattice and located as a highly
concentrated brine in the intercrystalline veins. As may be seen
from a photograph in the paper by Mulvaney et al (1988) the air
bubbles are intersected by these veins and films. This provides
conditions for diffusion of gas from bubbles to or from the intergranular
When alkaline dust neutralizes acids in the liquids contained
in ice, the resulting salt-H2O mixtures may cause freezing point
depressions. The extent of the freezing depression is dependent
on the types and concentrations of salts in the intercrystalline
brines; polyvalent ions will generally depress the freezing poinf
further than monovalent ions (Shepherd et al 1985). For example,
the experimentally determined freezing point depression for the
NaCl-CaCl2-H2O system was found to reach -51ºC at 30.59%
salinity (Oakes et al 1990).
From a low salinity water-salt solution, pure H2O ice will precipitate
with decreasing temperature, hence decreasing the volume and increasing
the salt concentration of the brine. In sea ice, for a temperature
range of -0.1 to -54ºC, a brine volume decreas by a factor
of 320 was found by Assur (1958). From nuclear magnetic resonance
and chemical analyses it has been found that such a brine can
maintain its liquid state down to -70ºC (Nelson and Thompson
1954; Richardson and Keller 1966; see also review by Weeks and
Ackley 1982). One may expect that, at greater depths in the Antarctic
ice sheet, where the temperature increases, for example, by 21ºC
at 2040 m depth at Vostok (Vostretsov et al 1984), and by 21ºC
at 1700 m depth at the Byrd station (Gow and Williamson 1976),
salinity and acidity will decrease as more H2O enters the liquid
phase. This will lead to a volume increase of intercrystalline
voids containing brine. As a first approximation, one may infer
from the data of Assur (1958) that this volume increase factor
could be near 100 at a depth of about 2000 m at Vostok, considering
a brine with similar ionic composition as that of standard sea
The depression of the freezing point of brine (salinity 27-35%o
permil) under a load pressure has a slope of about -0.0076ºC/bar
(Fujino et al 1974). Therefore, and because of increasing temperature
with depth, one may expect that deep in the ice sheet there will
be melting of the ice. On a macro-scale the brine migrates downward
in the ice sheets due to gravity and the density differences between
ice and brine. Under a temperature gradient the brine in the veins
will tend to migrate toward higher temperatures, ie downward,
because the brine density will be higher at lower than at higher
temperatures (Leeder et al 1987). The permeable ice acts as a
sieve for the brine, thereby redistributing elements, isotopes
The drainage may contribute in part to the formation of pockets
of lakes of liquid water below ice sheets, observed by radio-echo
sounding in Eastern Antarctica (Oswald and Robin 1973) and near
Vostok (Zotikov 1986). Budd et al (1971) calculated near basal
temperatures of -23.5ºC to -26.2ºC in the Vostok region.
Vostretsov et al (1984) measured temperatures in the Vostok borehole
down to a depth of about 2000 m. Linear extrapolation of these
latter data to a depth of 3700 m gives a basal temperature of
Oswald and Robin (1973) reported a marked increase in radar echo
strength of 10-20 dB (decibel) as the radar signals moved from
the ice to the sub-sheet lakes. The reflection coefficient fom
an ice-pure water interface would be around -3 dB, while the reflection
coefficient from an ice-rock interface would be -14 to -20 dB
(Oswald and Robin 1973). The increased radar echo strength means
that the radar signals were reflected from a medium with a much
higher conductivity than ice or pure water. This medium could
be saline water, moraine, clay, graphite-rich rocks, ore minerals,
or a mixture of such materials (Segalstad 1985). Because the radar
echo targets are extraordinarily smooth and have an area of up
to 8000 km^2 (Drewry 1987), they probably represent a substantial
layer of liquid water in the form of sub-sheet lakes (Oswald and
This water will have to be saline to be such a strong radar reflector.
Using the data of Fyfe (1964), Fujino et al (1974) and Potter
(1978) on freezing point depression versus salinity and pressure,
one can calculate that for a temperature of -4ºC in such
lakes the corresponding salinity should be about 4% (weight) NaCl
equivalents at the 3700 m basal ice depth at Vostok, under a pressure
of about 330 bars. The salt solution is not likely to consist
of dissolved NaCl only, but rather a mixture of different dissolved
mono- and multi-valent salts.
Gow et al (1968) detected pressurized liquid water at the ice-rock
interface beneath the ice sheet at the Byrd station at a depth
of 2164 m. Its chemical composition was not determined, but its
specific conductance of 45 micro-mho/cm^2 was higher than that
of melted ice (1.7-3.1 micro-mho/cm^2).
Changes in salt concentrations influence the solubility of gases
in the brine. Yasumishi and Yoshida (1979) found that the solubility
of CO2 in aqueous solutions of electrolytes commonly present in
polar ice decreases slightly with increasing concentration of
salts. The decreasing salinity and increasing intercrystalline
brine volume at great depths in the ice sheet may be partly responsible
for the observed variation in the CO2 content of the gas inclusions
from Antarctic cores.
The discovery of a liquid phase in ice at very low temperatures
is extremely important. It indicates that, in ice sheets, even
in the coldest regions, there is a quasi-infinite network of liquid-filled
veins in which dissolved substances and suspended micro-particles
can migrate and interact.
The use of Antarctic and Greenland ice cores for studies on the
chemical and isotopic composition of the atmosphere of past epochs
is based on the assumption that cold polar ice sheets lack a liquid
phase. Therefore, diffusion effects and mass transport in the
fluid phase were ignored, and it was generally accepted that Antarctic
ice can preserve unchanged the original composition of entrapped
gas for long periods of time. Recent studies have provided convincing
evidence that this fundamental assumption is wrong.
AND PHYSICAL-CHEMICAL PROCESSES
Changes caused by increasing load pressure in the ice sheets,
and by its relaxation in the ice cores, are of great importance
for understanding processes that influence the elemental and isotopic
composition of air inclusions in the ice. Some of the processes
are listed in Table 6, and their effects are illustrated in Figure
11. We discuss them in relation to concentrations of CO2 recorded
in the Vostok core. Most of the arguments for the Vostok core
are also valid for the CO2 record in a 1900 m deep Byrd core (Neftel
et al 1988) and for other deep cores.
A striking feature of the data shown in Fig. 11 is that the vertical
distribution of CO2 concentrations is similar to the distribution
of brittle cracks, pressure of air inclusions in relaxed cores,
core volume expansion, crystal size, and disappearance of primary
air bubbles and formation of secondary cavities. Such similarities
strongly suggest that the concentrations of CO2 in the gas recovered
from the Vostok core are artifacts caused by in situ and post-coring
A total disappearance of primary air bubbles in the ice sheet
was observed at the Byrd station by Gow et al (1968) and Gow (1971)
below a depth of 1200 m. The secondary gas cavities reappeared
in the bubble-free deep ice cores after the load pressure was
removed by drilling (Fig 11).
Miller (1969) postulated that the formation of air clathrates
causes the disappearance of air bubbles at greater depths in polar
ice sheets, and that the reappearance of the secondary gas cavities
is caused by dissociation of clathrates. He predicted that N2
and O2 clathrates start to form at a temperature of -30ºC
at depths of 850 and 600 m, respectively. This was confirmed by
Shoji and Langway (1982, 1983), who found that air clathrates
appeared at Dye 3 (Greenland) near a depth of 994 m. They also
found that secondary gas cavities, having irregular shapes, were
gradually forming by dissociation of single clathrate inclusions
during 86 and 180 hours after core cutting in the borehole. The
newly formed cavities grew exponentially with time, driven by
high O2 and N2 clathrate dissociation pressures in the decompressed
The N2/O2 composition ratio of the clathrates trapped in deep
ice cores collected at Dye 3 (Greenland) was 1.7, ie markedly
lower than the N2/O2 ratio of 4 in atmospheric air. This is because
oxygen molecules form hydrate clathrates earlier and at shallower
depth than nitrogen molecules (Nakahara et al 1988). This separation
mechanism changes the elemental composition of clathrates and
that of the gas in the air bubbles.
The depth compression of ice is relaxed elastically before the
cores reach the surface (Gow 1971). At the Byrd station this period
of relaxation results in a volume increase of about 0.2% for near-bottom
samples. Subsequent relaxation of the load pressure occurs in
the cores over a longer period, and is governed by the bubbliness
of the ice and orientation of the ice crystals.
This relaxation may cause up to 0.6% expansion of the volume
of the core (Fig 11). Gow (1971) found that the volume expansion
and decrease in ice density after relaxation was accompanied by
more than a two-fold dilation of pressurized air bubbles in which
disk-like cracks were formed intersecting the air bubbles. The
cores still continued to relax 27 months after they were drilled.
Sections of the Byrd core from depths between 400 and 900 m exhibited
brittle fractures. These stressed cores tended to fracture at
the slightest mechanical shock. The ice contained abundant air
bubbles (Fig 11), in which the air pressure in situ was between
40 and 80 bars, decreasing to about 15 bars 1 year after drilling
(Gow and Williamson 1975). In the Vostok core, fractured ice was
already present at a depth of 125 m (Legrand et al 1988).
An improvement in the mechanical condition of the Byrd core,
associated with the gradual disappearance of air bubbles, was
observed below 900 m depth. Some relaxation effects, such as volume
expansion and density decrease, diminished in the deeper core
sections (Fig 11) but other effects appeared. Three distinctive
types of inclusions were observed after the deep core sections
were brought to the surface: secondary gas cavities, plate-like
inclusions, and cleavage cracks. All three types are of secondary
origin, because none were observed in the initial examinations
immediately after the core sections were pulled to the surface,
but only after a longer period (Gow 1971) .
Secondary gas cavities started to appear in the Byrd core at
around 600 m depth. They were faceted in reflected light, had
a knobby external appearance, and were easily distinguished from
primary air bubbles, which possess smooth outlines. The secondary
cavities were filled with gas, the quantity of which increased
with time (Gow 1971). The cavities were surrounded by crystal
disturbances (Shoji and Langway 1983) possibly formed by quasi-explosive
(decrepitation) dissociation of air clathrates.
Cleavage cracks made their first appearance at about 600 m depth.
They were not uniformly distributed in cores, especially in the
deep ones. There the cleavage cracks were clumped together in
layers. The number of these cracks increased with time. Their
thickness was 1-10 micrometers, and their volume was about 10%
of that of the secondary cavities. In the very deep ice, below
2000 m, cleavage cracks were larger and more numerous. Plate-like
inclusions appeared after a period of storage of the core sections
from a depth below 800 m. They were 1.0 micrometers thick. The
formation of these voids can probably be attributed to strain-induced
vacancies (Gow 1971).
Gow and Williamson (1976) found that the crystal size in the
Antarctic ice sheet changes with depth in an irregular way. They
observed a constantly increasing crystal size from the firn-like
transition to a depth of 400 m, an irregular plateau in the size
distribution between 400 and 1200 m, a sharp decrease in size
with a peak near 1600 m, then a dramatic crystal size increase
at 1800 m and below (Fig 11). Similar observations were reported
from other Antarctic and Arctic locations (Herron et al 1985;
Langway et al 1988; see also review by Patterson 1981).
In a Dome C (Antarctica) core, between 400 and 550 m depth, Duval
and Lorius (1980) found a coincidence between a sudden decrease
in crystal size and a sudden change of delta-18O values in the
ice. They interpreted this as a climatic signal. However, they
were unable to offer a plausible mechanism to explain the relationship
between crystal size and climatic changes. We propose that these
delta-18O changes are due to recrystallization of ice in the ice
sheet and in the ice core. Crystal growth was also observed when
the ice cores were stored for a few days (Duval and Lorius 1980),
1 month, or 1 year (Narita and Nakawo 1985).
Later, Petit et al (1987) reiterated the idea of a correlation
between climate and ice crystal size in the Dome C (Antarctica)
core, but they noted that it suffers limitations due to in situ
processes. The temperature memory of crystals proposed by Petit
et al (1987) was criticized by Alley et al (1988). Alley et al
(1988) found that this memory lasts only hours rather than thousands
of years, and that the small crystal sizes in Wisconsinian ice
are probably caused by impurities in the ice. Langway et al (1988)
and Shoji and Langway (1989) observed an inverse correlation between
sulphate concentrations and crystal size in the Dye 3 core. The
reason for this variability remains unknown (Paterson 1981; Shoji
and Langway 1989).
The strikingly similar vertical distribution of CO2 and crystal
size in the Vostok core is more likely caused by physico-chemical
processes in the ice sheet and in the ice core than to past climatic
The first dramatic decrease in CO2 concentrations in the Vostok
core, at a depth of about 400 m, coincides with the region where
major brittle cracks appear (Fig 11) associated with disk-like
cracks intersecting the air bubbles. In this zone the relaxation
stresses exceeded the tensile strength of the ice. This part of
the core was characterized by an abundance of air bubbles with
a high initial (in the ice sheet) pressure of 34 bars, and by
a rapid increase of core volume after relaxation of pressure in
the bubbles to about 15 bars. At such an initial pressure and
at a temperature near -50ºC, gaseous CO2 enters the solid
clathrate form (Takenouchi and Kennedy 1965), but not other air
components (Miller 1969). During dry crushing of such samples
in the laboratory at low temperature, solid CO2 clathrate may
be left in the ice and may not be included in the gaseous CO2
recovered from the bubbles. This will lead to erroneously lower
The low CO2 concentrations near 750 m depth coincide with the
highest core volume expansion, accompanied by dramatic dilation
of air bubbles and by crack formation (Gow and Williamson 1975).
The dilation of bubbles was accompanied by a decrease in bubble
pressure. This must be associated with differential escape of
gas from the bubbles into the micro-cracks and liquid veins within
the ice, and also out of the cores. Depletion of CO2 is clearly
dependent on changes in ice structure, which are reflected in
core volume expansion, crystal size distribution, and disappearance
and reappearance of air inclusions (Fig 11). This depletion can
be due to lower viscosity and smaller molecular volume of CO2
than of other air components. Contamination by drilling fluid
was probably one of the factors influencing the changes in gaseous
components of the core.
Below about 900 m all original air bubbles disappear in the Vostok
core (Korotkevich et al 1978). All gas cavities from which CO2
was determined below this depth were secondary, and formed by
dissociation of N2 and O2 clathrates, and by degassing from crystals,
intercrystalline liquid, and contaminating drilling fluid. Both
these processes, and those that earlier caused the disappearance
of the air bubbles, must have led to chemical and isotopic fractionation
of the air components. The CO2 and deltaD changes in the Vostok
core, which are out of phase by about 1000 to 13,000 years, are
supposedly an artifact of this fractionation, rather than due
to ancient climatic fluctuations ass assumed by Barnola et al
The steady increase in CO2 concentrations between about 900 and
1876 m depth parallels the volume expansion curve, and that between
900 and 1500 m the secondary cavities' curve and the crystal size's
curve in Fig 11. The sudden decrease in CO2 concentrations below
1876 m in the Vostok core is probably due to slower release of
CO2 than of N2 and O2 from clathrates to poorly developed secondary
cavities. In cores from this depth, secondary cavities retain
much higher gas pressure than cavities in shallower cores.
The extent of contamination of the Vostok core by drilling fluid
and its impact on the gaseous composition of the core is different
at different depths, depending on the vertical distribution of
post-coring processes. In different ice cores the greatest contamination
occurred at similar depths (near 500 and 2000 m) at which abrupt
CO2 decreases were reported.
Ice is not a rigid material suitable for preserving the original
chemical and isotopic composition of atmospheric gas inclusions.
Carbon dioxide in ice is trapped mechanically by dissolution
in liquid water. A host of physico-chemical processes redistribute
CO2 and other air gases between gaseous, liquid and solid phases,
in the ice sheets in situ, and during drilling, transport and
storage of the ice cores. This leads to changes in the isotopic
and molecular composition of trapped air. The presence of liquid
water in ice at low temperatures is probably the most important
factor in the physico-chemical changes. The permeable ice sheet
with its capillary liquid network acts as a giant sieve which
redistributes elements, isotopes and micro-particles.
Carbon dioxide in glaciers is contained: (1) in interstitial
air in firn; (2) in air bubbles in ice; (3) in clathrates; (4)
as a solid solution in ice crystals; (5) dissolved in intercrystalline
veins and films of liquid brine; and (6) in dissolved and particulate
carbonates. Most of the CO2 is contained in ice crystals and liquids,
and less in air bubbles. In the ice cores it is also present in
(7) the secondary gas cavities, (8) the cracks, and (9) in the
traces of drilling fluid.
The concentration of CO2 in air recovered from the whole ice
is usually much higher than that in atmospheric air. This is due
to the higher solubility of this gas in cold water, which is 73.5
and 35 times higher than that of nitrogen and oxygen respectively.
The composition of other atmospheric gases (N2, O2, Ar) is also
different in ice and in air inclusions than in the atmosphere.
Argon-39 and 85 Kr data indicate that 36-100% of air recovered
from deep Antarctic ice cores is contaminated by recent atmospheric
air during field and laboratory processing.
Until about 1985, CO2 concentrations in gas recovered from primary
air bubbles and from secondary gas cavities in pre-industrial
and ancient ice were often reported to be much higher than in
the present atmosphere. After 1985, only concentrations below
the current atmospheric level were published. Our conclusion is
that both these high and low CO2 values doe not represent the
real atmospheric content of CO2.
Recently reported concentrations of CO2 in primary and secondary
gas inclusions from deep cores, covering about the last 160,000
years, are much below the current atmospheric level, although
several times during this period the surface temperature was 2-4.5ºC
higher than now. If these low concentrations of CO2 represented
real atmospheric levels, this would mean (1) that CO2 had not
influenced past climatic changes, and (2) that climatic changes
did not influence atmospheric CO2 levels. There are numerous other
contradictions between the CO2 records from deep Antarctic and
Greenland ice cores and stable isotopic records, and also between
the CO2 records from different sites.
In air bubbles from shallow ice cores, collected in Antarctica
just below the firn/ice transition depth, the CO2 concentrations
were found to be similar to those in the current atmosphere. The
ice at this depth was deposited in the 19th century. The CO2 concentrations
in these ice cores decreased proportionally to increasing depth
and load pressure. Such a vertical distribution of CO2 in shallow
cores has been incorrectly interpreted as indicating an anthropogenic
increase of atmospheric CO2 in the 19th and 20th century. This
interpretation was based on the speculative assumption that the
air in bubbles in ice deposited during the 19th century and earlier
is much younger than the ice, due to a free exchange of air contained
in the deep polar firn with the atmosphere. However, stratigraphic
studies and radioactive and stable isotope data demonstrate that
this assumption is not valid, because the air in firn is sealed
off by hundreds of ice layers.
In samples of gas from pre-industrial ice, CO2 concentrations
fluctuated by a factor of about 10, and were the same, considerably
higher, or lower than in the present atmosphere. The reported
analytical errors of CO2 measurements at a 95% probablility level
were of the same magnitude as the hypothesized recent rise in
atmospheric CO2 levels.
In deep cores the vertical distribution of CO2 represents the
effects of in situ physico-chemical processes, artifacts in cores,
and arbitrary rejection of high readings, rather than original
concentrations in an ancient atmosphere. The most important among
these processes are the dissolution and diffusion of gases in
intercrystalline liquids, the formation and dissociation of clathrates,
the disappearance of air bubbles and the formation of secondary
air cavities, the formation of macro- and micro-cracks, thermal
gradients during drilling, transportation and storage of cores,
and contamination of cores by drilling fluid. Contamination led
to fluctuations in heavy metal and Al readings in the cores, incorrectly
interpreted as being due to ancient climatic changes.
During the disappearance of air bubbles in ice sheets and the
formation of new secondary cavities in the ice cores, the chemical
and isotopic fractionation of gases occurs in two stages.
First, when the air bubbles disappear sympathetically with increasing
load pressure, the gas dissolves in liquid water and in ice crystals,
and condenses to solid-state clathrates. At various depth, pressure,
and time, the individual gases are transferred at different rates
from bubbles to ice crystals and to intercrystalline liquid veins
and films. In the liquids and crystals they may remain in solution
or as solid clathrates, mixed with gases which wer present there
before the disappearance of the bubbles. Isotope ratios and gas
composition are changed at each state transition (between ice,
liquid and vapour). Thus during this first stage the molecular
and isotopic composition of gases will change both in the air
bubbles and in the ice itself.
During the second stage, when the load pressure decreases in
the bubble-free ice cores, particular gases are successively released
from the ice crystals, liquids and clathrates, forming new secondary
gas cavities. During this process the state transitions are reversed,
ie from solid and liquid to gas. The contribution of gases that
were originally trapped in the ice crystals and liquids to the
air in the new cavities, remains to be determined. But it is highly
improbable that, following such dramatic changes, the gas in the
secondary cavities should have the same composition as in the
Changes in current and past atmospheric CO2 levels cannot be
established from the available data on CO2 in polar ice with any
confidence. Even the fundamental information on the relation between
CO2 concentration in the air contained in 10-year-old glacier
strata and in the contemporary atmosphere is not available. In
such a situation this relation cannot be realistically determined
for 100 or 100,000 year old ice, for example. More than 20 physical
and chemical processes in the ice sheets and in the ice cores
(listed in Table 6) make the gas samples recovered from both shallow
and deep cores unrepresentative of the original elemental and
isotopic composittion of paleo-atmospheres.
With the methodology currently used, the glaciers cannot divulge
the true CO2 and paleo-climatic story. The idea that this is the
case has dawned on CO2 students, who recently started to realize
that the CO2 variation observed in ice cores "is an artifact,
due to the enclosure process or ice-gas interaction" (Siegenthaler
et al 1988). But harm has already been done: in the past decade,
unidirectional and uncritical conclusions from ice core studies,
repeated in myriad publications, have led to a widely accepted
false tenet that the man-made climatic warming hypothesis is based
on solid ground. This dogma has already had political consequences,
and may have an enormous impact on the future economy and development
of society. But it was never critically evaluated in depth.
In order to study man's hypothetical impact on climate, of paramount
importance is information on changes in atmospheric composition
during the last 150 years, and not the prehistoric changes. We
therefore propose tha, in future CO2 studies of glaciers, the
emphasis should be placed on the firn and ice deposited in this
period. Drilling cores, even from shallow depths, is an extremely
brutal procedure leading to drastic changes in the ice samples,
precluding their reliability for gas analyses. Other methods of
sampling should be considered. Collecting the gas in situ may
possibly be the best of them. If firn and ice samples are to be
considered they should be sealed off at the moment of collection,
and their temperature kept constant until analysis. The gas should
be extracted from the ice at the sampling site during or immediately
after collecting the samples. The samples should be collected
from the ice sheet starting at the surface down to layers deposited
in the first half of the 19th century. Processes which change
the elemental composition of air inclusions in snow and ice should
be carefully studied before deciding whether the upper parts of
polar ice sheets can be used for reconstruction of the pre-industrial
ACKNOWLEDGEMENTS and REFERENCES:
please refer to online
After the present paper was submitted for publication we read
the paper by Wahlen et al (1991), in which the authors presented
results of CO2 measurements made in a 95 m long ice core, drilled
below the firn-ice transition in Greenland. The ice at the starting
depth was deposited in 1720, the CO2 concentration in this air
was 310 ppm. By assuming that the age of air was 220 years younger
than the age of ice, the authors smoothly connected their results
to the recent atmospheric observations from Mauna Loa. The criticism
and reservations for similar interpretation of ice core data in
sections SHALLOW CORES and AGE OF AIR IN ICE are valid also for
Engelbeen: Water in the ice
(a historical note of where
I started, trying to grasp FE's criticisms of Jaworowski.)
This is because the ice cores do not fulfill the essential
closed system criteria. One of them is a lack of liquid
water in ice, which could dramatically change the chemical
composition the air bubbles trapped between the ice
crystals. This criterion, is not met, as even the coldest
Antarctic ice (down to –73oC) contains liquid
Engelbeen: According to what
Jaworowski suggests, CO2 may go into that water, even
may be retained there during the measurements, which
leads to too low CO2 measurements. But the article where
he refers to is not about water, but about sulfuric
acid at the triple joints of ice crystals, away from
enclosed air bubbles . Moreover, concentrated sulfuric
acid may melt a tiny part of the ice, but it also makes
that little CO2 (if in contact at all) can be dissolved
in the liquid. Further, CO2 measurements are done by
crushing the ice at low temperatures, under high vacuum,
over a cold trap (-70°C), which effectively sucks
CO2 out of any liquid water...
Engelbeen has not noticed the irony of describing
a vacuum sucking CO2 out of liquid water, which is exactly
the same principle whereby Jaworowski et al suggest that
CO2 CAN migrate upwards - under negative pressure from above!
Engelbeen's source  is an activist blog "Some Are
Boojums". With a name like that I would, on principle,
not trust its science. Reading further, it becomes clear
that the author has really got it in for Jaworowski. Admittedly,
Engelbeen says that Mulvaney is not available on the Internet,
so perhaps S-A-B is a distasteful necessity. But the evidence
S-A-B provides is Mulvaney on sulphuric acid in ice, which
is evidence Jaworowski himself refers to! This is strange
if it is meant to disqualify Jaworowski - but not impossible.
But Engelbeen talks about concentrated sulphuric acid, which
simply doesn't exist in ice cores. And in the very diluted
form of sulphuric acid we have in the ice lattice, the solubility
of CO2 is not going to change appreciably. [later findings
confirm this but sorry, ref. mislaid!] Lastly, there is
no meaning in Engelbeen talking about "triple joints
in ice crystals away from the enclosed air bubbles"
as we are dealing with a vast capillary network along all
ice crystal joints and edges of all bubbles, even at -70C.
There is not the slightest
evidence that liquids in the ice at the extreme cold
temperatures in Antarctica play any role in the CO2
values measured. Moreover, different ice cores of Antarctica
with completely different profiles near the coast (high
accumulation, more dust/salt inclusions, less cold temperatures),
compared to the more inland ice cores, show the same
low CO2 values (within 5 ppmv) for overlapping periods
of the same gas age. There is no evidence that liquids
in the ice structure lead to underestimated CO2 values
for the Antarctic ice cores.
I'd like to see the ice core evidence (that completely
different profiles show the same low CO2 values) because
I've heard too many such statements that did not bear
out on closer examination. Perhaps Engelbeen is right,
but I want to check further, not for the lack of evidence
that "liquids in the ice... play any role in the
CO2 values measured" but for the positive existence
of evidence that even examines this issue of water films....
[after this point, I returned to Jaworowski and after
reading it all, I felt that all Engelbeen's subsequent
points were well answered in Jaworowski's text.]